Giving training boost to young player when playing next to experienced player with lots of league,IE and international games under his belt.

Keeping the boost specific to the line for example experienced Midfielder giving boost in midfield and vice a versa

This can fix the problem of all trainer clubs and force club to bring experienced player to train the youth

I would suggest decreasing the value of gain through training season and increasing it in games played.

Also having more experience gains when playing league
Highest xp gain when playing international or Champions league

High XP gains based on current level of team